In order to be used in packing applications, biaxially oriented polypropylene (BOPP) films should have excellent mechanical properties, excellent barrier properties (of moisture vapor and/or oxygen gas), and excellent heat sealability among other properties. In addition, BOPP films can include a promoting layer(s) suitable for various laminating, coating, and printing processes.
For some packaging applications, BOPP films should have both good moisture vapor and oxygen gas barrier properties as well as transparency. A high barrier to moisture vapor and oxygen gas can maintain product quality in an extended shelf life, whereas transparency provides consumers with visibility to the commodity product in packaging.
Metallization of BOPP films via vacuum deposition of aluminum is a well-known cost-effective method to significantly improve the moisture and oxygen barrier properties of BOPP films; however, the resultant films are opaque. Therefore, this technology is not suitable for packaging applications that require transparency.
U.S. Pat. No. 6,788,379 describes a plasma oxide coating process via enhanced chemical vapor deposition on different polymer sheets. The resultant oxide coated sheets demonstrated significantly reduced transmission rate for moisture vapor and oxygen gas. The process of aluminum oxide (AlOx) or silicon oxide (SiOx) coating of BOPP films via roll to roll web vacuum deposition provides a transparent thin coating layer of aluminum oxide or silicon oxide on the substrate that gives the packaging materials high moisture and oxygen gas barrier properties as well as transparency. However, AlOx or SiOx coating is extremely brittle and prone to cracking of the oxide coatings occurring in the downstream processes of converting and packaging applications, such that the high barrier properties of the packaging cannot be maintained due to said cracking. In addition, the cost of such oxide coating process is comparatively high and uncompetitive to most other high gas barrier OPP film technologies.
U.S. Pat. No. 5,672,426 describes a process of making a transparent liquid crystalline polymer (LCP) film via solvent casting. The LCP is dissolved in a solvent to an appropriate concentration and then cast into a film. Although cast LCP films have good transparency, excellent barrier properties for both moisture vapor and oxygen gas, and excellent mechanical properties, the solvent casting process is very costly and not environmentally friendly. Melt-extrudable LCPs have been known to present a processing difficulty in making LCP films. Limitations of LCP film extrusion include high temperature for processing, low melt viscosity, poor adhesion to substrates, broad variations in thickness, opaqueness to light, and uni-axial orientation due to the unique arrangements of LCP chains in the materials. Thus, the use of LCP packaging films is restricted to the applications of specialty packaging since both materials and processes are relatively expensive for general industrial packaging.
U.S. Pat. No. 5,139,878 describes a multilayer film structure comprising at least one fluoropolymer layer laminated to polyolefin or polyester layer via a tie layer or adhesive layer through a process of coextrusion or lamination. Such a resultant multilayer film exhibits high moisture vapor barrier and good transparency. However, the oxygen barrier of the resultant film is only slightly improved over the polyester film of similar thickness. Most fluoropolymers especially polychlorotrifluoroethylene (PCTFE) exhibit excellent moisture barrier properties (moisture transmission rates in the range of from 0.02 to 0.06 g/100 in2/day) and transparency; however, the fluoropolymers usually do not provide significant oxygen gas barrier properties. Fluoropolymer films are restricted to use in specialty packaging since both fluoropolymers and equipment used for fluoropolymer film production are very expensive. In addition, fluoropolymer films during incineration generate hydrogen fluoride (HF) which is an environmentally hazardous byproduct.
BOPP films coated with polyvinylidene chloride (PVdC) exhibit both good moisture and oxygen barrier properties as well as good transparency as the PVdC coating reaches a thickness level required for the barrier properties. A few microns of PVdC coating on BOPP substrate is usually needed to achieve good moisture vapor and oxygen barrier properties. A costly offline coating process is needed to apply a thick PVdC coating on a BOPP substrate. Although PVdC coating is a good candidate for moisture vapor and oxygen gas, it also has well-known environmental issues. Coated BOPP films cannot be recycled into the reclaim streams of BOPP film production due to the intrinsic characteristics of PVdC materials. Furthermore, PVdC coating in packaging materials decomposes into hazardous substances (for instance hydrogen chloride, HCl) as PVdC coated films are burned by waste incinerators.
U.S. Pat. No. 5,155,160 describes a polyolefin film structure comprising a precisely loaded paraffin wax additive in the range of 3 to 10 wt % based on the weight of the polyolefin. The resultant film was claimed to have improved moisture vapor barrier. The film needed to be quenched immediately at a chill roll temperature of about 4° C. after it is extruded to avoid wax migration. Since the molecular weight of the paraffin waxes was in the low end range of 300 to 450 g/mole, while the loading in the film was at a high end range of 3 to 10 wt %. Inevitably, the paraffin waxes may migrate onto the surface of the resultant film in downstream processes such as orientation in machine and transverse direction, converting operations, and packaging applications, which may lead to wax plate-out problems and contamination of processing equipment. In addition, the resultant film showed very limited improvements in oxygen barrier.
U.S. Pat. No. 6,033,514 describes a biaxially oriented multilayered polypropylene film with improved moisture vapor transmission rates (MVTR) by incorporating a crystalline polyethylene wax additive into a core resin layer. The content of wax additive is in the range of 1.1 to 7.5 wt %. A single core resin layer is extruded without outer layers, coated with two polyolefin cap layers after the orientation in machine direction, and oriented in the transverse direction. The barrier mechanism of the film is claimed to be that the wax additive in the core layer migrated throughout the polyolefin cap layers to form highly-crystalline continuous wax layers on the outer surfaces of the polyolefin cap layers. In the cases of coextruded outer layers or tie layers, all given examples showed no improvements in moisture vapor transmission rates. In a production line, it is not efficient to add two cap layers onto a single core layer at the stages in-between the two orientation steps. Although the outer polyolefin cap layers delayed the rate of wax migration, the wax additive in the core layer will continue to migrate through the polyolefin cap layers and form wax crystals on the surface, which may cause processing issues downstream such as wax plate-out or build-up. In addition, the resulting film showed very limited capability to improve oxygen gas barrier which is required for some food packaging applications.
U.S. Pat. No. 6,033,771 describes the use of waxes to improve moisture and oxygen barrier properties of multilayer BOPP films. In this patent, 4.5 wt % Fischer Tropsch wax is blended into a polypropylene core layer which is cavitated to form voids. The voids in the core layer are used to entrap the wax molecules to prevent them to migrate to the surface, thus avoiding plate-out problems. However, the cavitation of the intermediate layer renders the resulting film opaque and no longer transparent.